The present invention relates to a portable device having a charging circuit for charging a secondary rechargeable battery incorporated in a detachable battery pack and to a semiconductor device configuring the charging circuit.
Portable devices, such as a notebook computer, a personal digital assistant (PDA), and a cellular phone, use battery packs. A battery pack includes a secondary battery. A lithium ion battery is often employed as the secondary battery. The lithium ion battery is advantageous in that it decreases the operation cost of devices using the battery and in that it enables a large amount of current to be instantaneously discharged. A device employing a secondary battery, such as a lithium ion battery, normally incorporates a charging circuit. The charging circuit is connected to an AC adapter, which functions as an external power supply, to charge the secondary battery. However, recent portable devices have more functions and are becoming more compact. As a result, the charging circuit is required to be more compact and must be fully charged more quickly.
When using a lithium ion battery in a portable device, the charging capacity of the battery is greatly affected by the charging voltage. Thus, the charging voltage must be accurately controlled. Accordingly, the battery is charged by a constant voltage and a constant current. The lithium ion battery is sensitive to overcharging and overdischarging. The battery deteriorates when the charging voltage is too high, and overdischarging makes it difficult for the battery to function properly. Accordingly, in addition to the lithium ion battery, a battery pack for a portable device includes a protection circuit, which prevents overcharging and overdischarging of the battery.
When charging a lithium ion battery in a battery pack, a charger controls the charging voltage supplied to the battery pack to be at a desired voltage value. The protection circuit of the battery pack includes a switch circuit for preventing overcharging and overdischarging. Due to the impedance of the switch circuit, the actual voltage applied to a battery cell in the battery pack is lower than the charging voltage that the charger supplies the battery pack with. This results in shortcomings such as insufficient charging and lengthened charging time. To solve such a problem, Japanese Laid-Open Patent Publication 11-187588 (pages 4 and 5, FIG. 1) describes a method for directly detecting the voltage of the battery cell in the battery pack to control charging in accordance with the detected voltage value.
FIG. 1 is a schematic circuit diagram showing a charging system 100, which is described in the publication. As shown in FIG. 1, a battery pack 42 is connected to a charging circuit 41. The battery pack 42 includes a battery cell 43, a protection circuit 44, a battery protection resistor 45, a positive terminal t1, a negative terminal t2, and a voltage detection terminal t3 of the battery cell 43. The charging circuit 41 includes a power supply unit 46, a reverse flow prevention diode 47, an output control transistor 48, cell voltage detection resistors 49, 50, 51, and 52, a current detection resistor 53, an operational amplifier 54, and a charge controller 55.
In the charging circuit 41, the cell voltage detection resistors 49 and 50 are connected in series between the positive terminal t1 and the negative terminal t2. The resistors 49 and 50 divide the voltage between the positive terminal t1 and the negative terminal t2 of the battery pack 42. A voltage dividing node is connected to a non-inverting input terminal of the operational amplifier 54.
The cell voltage detection resistors 51 and 52 are connected in series between the voltage detection terminal t3 and the output terminal of the operational amplifier 54. A connection node between the resistors 51 and 52 is connected to the inverting input terminal of the operational amplifier 54. The cell voltage detection resistors 49 to 52, the battery protection resistor 45, and the operational amplifier 54 configures a differential amplification circuit. Each resistance of the differential amplification circuit is adjusted to obtain the amplification rate of “1”. This supplies cell voltage from the operational amplifier 54 to the charge controller 55.
The charge controller 55 retrieves the potentials at both ends of the current detection resistor 53 and detects the charging current from the voltage drop of the resistor 53. The charge controller 55 controls the output control transistor 48 based on the charging current and the cell voltage, which is supplied from the operational amplifier 54, to charge the battery cell 43 at a constant current and constant voltage. More specifically, the battery cell 43 is charged by a constant current until reaching a predetermined charging voltage. After reaching the predetermined charging voltage, the battery cell 43 is charged by a constant voltage. In this manner, the charging voltage, which is obtained by correcting the voltage drop at the protection circuit 44, is applied to the battery cell 43, and the voltage of the battery cell 43 is increased. As a result, the battery is sufficiently charged and the charging time is reduced.
The battery pack 42 is detachable. Thus, there is always the possibility of contact failure with the charging circuit 41. In the charging system 100, when there is a contact failure at the voltage detection terminal t3, which detects the voltage of the battery cell 43, the cell voltage is not correctly detected. This hinders charging with the constant voltage. In other words, even if the battery cell 43 is charged with the constant current until the charging voltage reaches the predetermined value, charging cannot be switched from the constant current to the constant voltage. Thus, charging is continued with the constant current. This may increase the charging voltage of the battery and overcharge the battery cell 43.